The invention relates to an apparatus and method for detecting overlapped paper sheet feed by discriminating a voltage detected by a photo sensor and a load resistor located in a paper sheet feed path, which represents a light transmittivity of paper sheet.
Various apparatuses have been developed which detect a misfeed of paper sheets in a paper feed path. "Misfeed of paper sheets" in the specification means that plural paper sheets are overlappingly supplied from a sheet stacker to the paper sheet feed path. A practical method for detecting the misfeed of paper sheets is to detect a thickness of paper sheet(s) supplied into the paper sheet feed path.
The thickness of a paper sheet has been measured by sensing a transmitted light through the paper sheet. Japanese Patent Application 61-189291 (Published Unexamined Patent Application 63-47244) disclosed two sets of paper sensors positioned at an exit of a paper stacker and an exit of a toner fixing station, respectively, of a copying machine. The paper sensor includes a light emitting diode (LED) and a photo sensor. The voltage applied to the LED is increased until the photo sensor detects the transmitted light through the paper sheet. When the photo sensor does not sense the transmitted light even when the voltage is increased to a maximum value, a control device judges that the thickness of the paper is too large for the copying machine. When the photo sensor senses the transmitted light and the voltage value is at a minimum value, the control device judges that the thickness of the paper is too small. When the voltage sensed by the photo sensor is in a nominal range, the voltage value sensed by the sensor at the exit of the paper stacker is stored and the voltage applied to the LED of each paper sensor is fixed. A voltage sensed by the photo sensor at the exit of the toner fixing station is compared to the stored value to determine the occurrence of a paper jam between the two sets of paper sensors. The Patent Application 61-189291 does not intend to sense the overlapped paper sheet feed of various thickness.
Japanese Patent Application 61-72167 (Published Unexamined Patent Application 62-229389) discloses a sensor for sensing a thickness of a bill, wherein various different threshold values for the various thickness of the bill are used, and one of the threshold values is used depending upon the results of the detecting operations. The Patent Application 61-72167 requires switching of the threshold values depending upon the results of detection.